In the art of encapsulating and bonding of printed circuit boards together to form multiple layers of printed circuit boards (hereinafter and in the claims sometimes termed multi-layer PCB'S) it is standard practice to bond the multiple layers of printed circuit boards by means of a partially cured glass-reinforced epoxy resin layer. Such a reinforced resin layer is termed a prepreg in the art and takes this name from the fact that a woven glass layer of a certain mesh size and thickness is pre-impregnated with a solution including primarily epoxy resin, catalysts and/or curing agents and other modifiers therefore, and then heated to only partially cure, i.e., B-stage, the particular epoxy resin system.
The resulting B-staged prepreg sheet material is a solid, at room temperature, has a predetermined resin thickness, a predetermined gel time, and predetermined resin flow under predetermined curing conditions of time, temperature and pressure upon final lamination of the multi-layer boards.
In order to control resin flow under curing conditions, standard prepreg sheets are made with varying ratios of resin (e.g., epoxy resin) to glass cloth, the maximum amount of epoxy resin being about 83% by weight, where maximum amount of resin fill of the copper circuitry is desired; the minimum amount of resin utilized is about 40% by weight, with the balance being essentially glass cloth, where a substantially different dielectric effect may be desired. Resin flow from the prepreg during the standard lamination and curing cycle (345.degree. F. at 200 psi for 45 minutes) can thus be designed to vary from a low of about less than 2% to as high as about 50-54% depending upon the ratio of resin to glass cloth. Resin flow % is that percentage of weight loss of resin from the original prepreg layer, when subjected to standard test conditions, e.g., in accordance with MILG-55636A specifications. Such specifications include subjecting the prepreg material to a pressure of 200 psi, for 45 minutes at 340.degree. F.
The amount of resin flow of the prepreg during the curing and lamination cycle is critical. Too little resin flow may not completely encapsulate dense copper layers of printed circuitry on the PCB. Too much resin flow will alter the thickness of the prepreg and the dielectric characteristics. The possibility of creating voids is particularly great where the metal circuit line or layer, e.g., of copper, is laid down on the circuit board in thick layers and/or is of high density (e.g., as is required for grounding circuits) making it extremely difficult for the resin of the prepreg material to flow completely into the deep corners formed between circuit board and the printed circuit details. Such a void condition is shown in FIG. 1 of the drawings.
Referring now to FIG. 1, a glass reinforced B-staged, epoxy resin system prepreg layer D of the prior art is there shown, in a laminated and cured state, placed between an epoxy-glass printed circuit board (PCB) layer B.sub.1, and a second PCB designated B. The layer B.sub.1 may be a so-called exterior, or cap, layer and may or may not carry circuitry details thereon. The PCB layer B carries, on one side thereof, a plurality of conductors A of relatively great thickness. In the circuit configuration shown in FIG. 1, voids C are formed between conductors A and the PCB layer B because the resin of the prepreg layer D cannot flow sufficiently into corners, to completely encapsulate the conductors A, even where a maximum resin flow type of prepreg is utilized, because of the presence of the glass cloth reinforcing layer E. The presence of voids can result in moisture penetration, circuit deterioration, and ultimately loss of circuit integrity.
In order to overcome the presence of voids during the manufacture of multi-layer circuitry, some manufacturers, prior to the lamination of alternating layers of multiple printed circuit boards and prepreg layers, have first flowed onto the PCB whose circuit is to be encapsulated, a pure or substantially pure liquid B-staged, resin system of a type that is compatible with the material of the circuit board. Such a resin, for example, may be a liquid epoxy resin system that has not been cured to any significant extent. Such a resin system is necessarily a viscous liquid, at room temperature, in order that the PCB may be readily adherently coated. The liquid resin-coated PCB is then placed between standard prepreg layers and laminated under temperature and pressure sufficient to cure the prepreg layers and the resin coating in the usual manner.
While voids may be eliminated from the circuitry of multi-layer PCB by the just-described method, the procedure is (1) difficult to work with, (2) time consuming, and, furthermore, (3) does not enable one to retain close control of dielectric separation, parallelism and thickness uniformity in the finished multi-layer PCB.
The major objective of this invention is to provide a simplified process for the bonding of i.e., laminating, multiple layers of PCB's by means of standard prepreg layers in which the possibility of voids adjacent the circuitry of the PCB layers is substantially eliminated, without significantly altering the dielectric properties, parallelism and thickness uniformity provided by the standard prepreg layers.